The invention pertains to servo controls for hydraulic actuators and more particularly to control systems that sense failure conditions and in response thereto assume a passive failure mode.
In the field of hydraulic control, there are numerous applications for hydraulic actuators for positioning mechanisms in response to command signals. Actuators used in certain critical environments, such as flight controls on aircraft, are often accompanied by various safety and backup systems which ideally protect against, or minimize the danger that may result from a failure of the actuator or an associated component including the fluid supply and return lines. For example, in flight controls, safety subsystems are known for counteracting "hard over" actuator commands resulting from a malfunction of one of the components or subsystems. A failure of one of the valves in a four-way valve that controls the position of an actuator may result in the actuator being driven "hard over" to one of its extreme positions. This in turn can cause the control surfaces of the aircraft to likewise be driven "hard over," flying the aircraft in an unintended and possibly dangerous direction.
To avoid this "hard over" failure mode, the usual safety systems provide for monitoring the actuator and associated controls and, if an error is detected, then backup or counteracting controls are activated to "fight" the failed actuator or the erroneous signal. These known systems typically include various electronic monitoring devices and associated electronic hardware and are themselves susceptible to a variety of possible failures. Thus, if the hydraulics of the actuator fans, and the electronic backup also fails, a "hard over" actuator condition may result because of the failure of the counteracting control or signal to neutralize the faulty command.